Coextruded structures containing two or more different layers are becoming more commonplace, especially in the food packaging industry. Multilayer plastic structures are displacing many materials, such as glass and metal containers, due to cost and performance factors. One route frequently used in the production of plastic containers is thermoforming from coextruded sheet. A typical structure will have an oxygen barrier layer to extend shelf life, outer or cap layers for moisture, taste and/or odor barrier purposes, and adhesive layers to bond the barrier layer to the outer layers. This type of production method can easily result in scrap levels of more than 50%. Since some of the materials are very expensive and the scrap material, or regrind as it is often called, can only be sold for a fraction of the cost of the original materials, as much of the regrind as possible is reused in the structure in order to make these plastic containers cost competitive with glass and metal containers. This regrind is typically used as an intermediate layer between the outer protective layer and the barrier layer and typically displaces a portion of the material of the outer protective layer. The regrind layer can be used symmetrically about a center layer (typically the barrier material) or on one side denoted as an asymmetric structure.
Heretofore, extruded plastic, especially multilayer polyolefin based, sheets have been subject to problems of delamination or separation within the regrind layers and/or turbulence, ripples or discontinuities between the layers. The problem is related to the physical properties of the regrind which is also related to its composition. Typical, for example, is regrind which contains polypropylene, ethylene vinyl alcohol copolymers (EVOH) and adhesive, examples of which are shown in FIGS. 1 and 2. The level of these components in the regrind is dependent on the loading of the individual components in the sheet structure. It is also dependent on the amount of regrind recycled; that is, as more regrind is recycled by displacing the polypropylene outer layers, the higher the level of EVOH and adhesive in the regrind. Such typical regrind has materials of different viscosities which can cause the problems stated above and detailed in the following paragraphs.
Delamination causes (a) unsightly defects where it is obvious that the sheet delaminated, (b) loss of part stiffness and (c) loss of impact resistance. Delamination can occur when materials of different viscosities and compatibility are not sufficiently mixed and/or the incompatible component(s) is present in too high a concentration to be dispersed into fine enough domains. At high shear rates, the higher viscosity materials tend to move toward the center of the layer. In ethylene vinyl alcohol copolymer (EVOH) films, this is believed to be due to a phase separation of the incompatible polar EVOH and the non-polar polyolefins in the high shear field inside the extruders, feedblock and/or die. In many cases, as with the use of EVOH, delamination in the coextruded material layers normally occurs during forming; delamination can also occur during flexing while in use before the desired lifetime of the product has expired.
Turbulence, which is a descriptive term rather than related to the classical definition of turbulent flow, can be caused by several factors in sheet coextrusion: moisture in the raw materials, irregular flow surfaces in the feedblock and die, or materials properties of the various components. Under normal processing conditions and with standard tooling, only the latter case, material properties, i.e., the mismatch in properties, is of any consequence. This type of turbulence seems to be related to the difference in viscoelastic properties between the outer cap layer(s) and inner, regrind layer(s).
The scrap material or regrind is generally formed into sheet by regrinding it, melting it and re-extruding it. The structure wherein a regrind layer is interposed between an outer layer and an inner layer suffers from the disadvantage that as more regrind is used in the laminate, the tendency towards turbulence and/or delamination is increased. As more regrind is added to the structure displacing the outer layer, a critical shear stress is reached at which point the interface between the two layers becomes unstable resulting in turbulence. The instability caused by this turbulence causes poor sheet quality and containers formed from this sheet are unacceptable in appearance and, when used with barrier materials, the barrier protection can be adversely affected, increasing oxygen permeability by as much as 10 times. The point at which the interface becomes unstable and turbulence occurs is dependent on the magnitude of the difference between the outer cap layer and the inner regrind layer, i.e., on the type of materials used. A case in point follows.
Viscosity data for polypropylene (nominal 2 MF) used as the cap layers, and two different regrinds generated from a barrier sheet containing a maleic anhydride-modified polypropylene adhesive and a 5.5 MF (@190.degree. C.) EVOH is shown in Table 1 below.
TABLE 1 ______________________________________ Shear Rate Viscosity (poise @ 200.degree. C.) (1/sec) PP (2 MF) Regrind A Regrind B PP (5 MF) ______________________________________ 20 21,500 13,500 13,000 15,000 100 8,400 5,500 5,000 5,700 800 1,600 1,550 1,300 1,500 2,000 1,100 750 700 750 ______________________________________
The adhesive and EVOH in the regrind cause it to have a significantly different viscosity than the polypropylene outer layers. At the same loading and processing conditions, the sheet structure utilizing Regrind A had no turbulence while the sheet containing Regrind B did show some rippling or discontinuous waves (i.e., turbulence). To determine if turbulence was truly a viscoelastic affect and not some type of anomaly associated with the regrind, the regrind layer was substituted with a polypropylene with similar viscosity to the regrind. A critical shear stress was reached at which point the interface became unstable.
As stated above, turbulence seems to be a viscoelastic dependent problem which increases as the amount of regrind used increases. Since from a cost standpoint it is desirable to use more regrind, there is a need for a method which allows the incorporation of a maximum amount of regrind but which prevents the onset of turbulence which leads to the problems discussed above and possibly also to delamination.
The present invention provides a method for making a laminate which can be used to make articles which include a greater amount of regrind material than was possible before but which also avoid the turbulence problems which would make the articles undesirable for use in most applications. This is accomplished by incorporating between the outer layer and the regrind layer a buffer layer which is formed of a blend of regrind material and the material of the outer layer.